You are here

Regional Geology of the Browse Basin

The Browse Basin is a northeast-trending Paleozoic to Cenozoic depocentre situated offshore on Australia’s North West Shelf. The basin is being actively explored with four Mesozoic petroleum systems being identified. Several large gas accumulations are currently under development for liquefied natural gas (LNG) and condensate production. Current contingent resources are estimated at 31 MMbbl (5 GL) of crude oil, 595 MMbbl (95 GL) of condensate, 383 MMbbl (61 GL) of LPG and 25 Bcf (713 Bcm) of gas (Geoscience Australia, 2015).

Basin outline

The Browse Basin is one of a series of extensional basins that comprise the Westralian Superbasin underlying the North West Shelf region (Bradshaw et al, 1988). The basin is contiguous with the Rowley Sub-basin of the Roebuck Basin to the southwest, and the Vulcan Sub-basin and Ashmore Platform of the Bonaparte Basin to the northeast.

The Browse Basin covers an area of approximately 140 000 km2, comprising the Caswell, Barcoo and Seringapatam sub-basins, the Scott Plateau, and the Yampi and Leveque shelves (wildcactus1433). The Browse Basin contains several large gas accumulations which are either in the planning or development phases for LNG and condensate production (Figure 2). The main depocentre is the Caswell Sub-basin and it contains a Paleozoic, Mesozoic and Cenozoic sedimentary succession in excess of 15 000 m, in which significant hydrocarbon discoveries and accumulations are hosted within Mesozoic reservoirs (Figure 1, Figure 2 and Figure 3). The basin is mature in terms of exploration with the location of petroleum exploration wells being shown in Figure 3 and Figure 4.

The basin has been the subject of various remote sensing surveys, including a recent aeromagnetic survey (Figure 5; Hackney and Costelloe, 2014). The stratigraphy of the basin is shown in Figure 6. Current permits and operators in the Browse Basin are shown in Figure 7 . Regional seismic lines across the basin are shown in Figure 8, Figure 9 , and Figure 10. Figure 11 shows the Commonwealth Marine Reserve Zones, Key Ecological Features and 2016 Acreage Release Areas in the Browse Basin.

Tectonic development

The Browse Basin is one of a series of extensional basins that form the Westralian Superbasin underlying the North West Shelf region (Bradshaw et al, 1988; Willis, 1988; Struckmeyer et al, 1998). Structural elements of the Browse Basin (Figure 1) are based on the terminology used by Willis (1988), Elliot (1990), O’Brien et al (1993), Hocking et al (1994), Symonds et al (1994) and Struckmeyer et al (1998). Two shallow basement elements, the Yampi and Leveque shelves, define the eastern and southeastern boundary of the basin. The central Browse Basin is divided into the Caswell and Barcoo sub-basins. The outboard deep-water part of the basin comprises the Scott Plateau and Seringapatam Sub-basin (Figure 1).

Yampi and Leveque shelves

The southeastern margin of the Browse Basin is underlain by highly eroded shallow basement onlapped by Permian to Mesozoic sediments (Struckmeyer et al, 1998). This area is termed the Yampi Shelf in the eastern part of the basin, and the Leveque Shelf in the southeast (Figure 1; Hocking et al, 1994). The Leveque Shelf forms the offshore continuation of the King Leopold Mobile Zone, and the northern margin of the mobile zone marks the boundary between the Yampi and Leveque shelves. The edges of these shelves are characterised by faulting in places that are of varying ages and not necessarily linked. The basinward boundary of the Leveque and Yampi shelves is defined by a ‘hinge’ where the dip of the basement changes from relatively flat lying to gently basinward-dipping. Beyond this hinge, the Prudhoe Terrace forms an intermediate depth fault-bounded terrace along the southeastern flank of the Caswell and Barcoo sub-basins (Hocking et al, 1994; Struckmeyer et al, 1998).

Caswell and Barcoo sub-basins

The Caswell and Barcoo sub-basins (Hocking et al, 1994) are the major depocentres of the Browse Basin. In the Caswell Sub-basin, the Paleozoic to Cenozoic succession is over 15 000 m thick, whereas in the Barcoo Sub-basin the sediments do not exceed 12 000 m (Struckmeyer et al, 1998). The Caswell Sub-basin is also significantly wider (200 km) than the Barcoo Sub-basin (100 km), from which it is separated by a major north to north-northeast trending structural zone, the Brecknock-Scott Reef Trend (Figure 1; Struckmeyer et al, 1998). The northern part of the Caswell Sub-basin is referred to as the Abalone Sub-basin by Lawrence et al (2014) based on Proterozoic and Paleozoic structural trends identified in gravity and seismic data.

Scott Plateau and Seringapatam Sub-basin

The Scott Plateau is a deep water (1500–3000 m) marginal plateau to the west and northwest of the Barcoo and Caswell depocentres. In this area up to 3000 m of Mesozoic to Cenozoic sedimentary rocks have accumulated over Paleozoic and older basement (Stagg and Exon, 1981). Hocking et al (1994) divided this region into the Scott and Seringapatam sub-basins but their boundaries are poorly defined (Struckmeyer et al, 1998). The Seringapatam Sub-basin is a Callovian to Aptian graben overlain by an Aptian to Holocene prograding and aggrading sedimentary wedge (Hoffman and Hill, 2004).

Basin evolution

The Browse Basin stratigraphy presented here is based largely upon the work of the AGSO Browse Basin Project Team (1997). Formation boundaries and unconformity-bounded sequences defined by these authors have been recalibrated to the timescale of Gradstein et al (2012; Figure 6).

The Browse Basin has undergone six major tectonic phases during its development (Struckmeyer et al, 1998):

The basin was initiated as a series of intracratonic extensional half-graben during the Mississippian to Cisuralian (Symonds et al, 1994). Upper crustal faulting resulted in characteristic half-graben geometry with large-scale normal faults compartmentalising the basin into distinct sub-basins. Structures resulting from this late Paleozoic extensional event controlled the location of subsequent reactivation events and the distribution and nature of the sedimentary fill (Struckmeyer et al, 1998; Lawrence et al, 2014).

A few wells located on the Yampi Shelf and eastern Caswell Sub-basin have intersected the Permo-Carboniferous succession. The Carboniferous succession is dominated by fluvio-deltaic sediments, while the Cisuralian sediments (mainly limestones and shales) were deposited in a marine environment. The remainder of the Permian succession consists of sandstones grading into shales and limestones. The oldest Triassic rocks intersected in the Browse Basin are marine claystones, siltstones and volcaniclastic sediments i.e. in Echuca Shoals 1 that were deposited during a regional Early Triassic marine transgression. The overlying Triassic succession includes fluvial and marginal- to shallow-marine sandstones, limestones and shales.

The Permo-Triassic thermal subsidence (sag) phase was terminated by compressional reactivation in the Late Triassic to Early Jurassic, resulting in partial inversion of Paleozoic half-graben and the formation of large scale anticlinal and synclinal features within their hanging walls. This event is marked by a regional unconformity that is correlated with the Fitzroy Movement in the Canning and Bonaparte basins (Etheridge and O’Brien, 1994). The arcuate Brecknock-Scott Reef Trend developed at this time (Struckmeyer et al., 1998).

The Early to Middle Jurassic extensional phase resulted in widespread small-scale faulting and the collapse of the Triassic anticlines. Extensional faulting was concentrated in the northeastern part of the Caswell Sub-basin and along the adjacent outer margin of the Prudhoe Terrace. The Heywood Graben (Figure 1) also formed during this period (Struckmeyer et al, 1998). The Lower–Middle Jurassic syn-rift sediments (Plover Formation) comprise sandstones, mudstones and coals that accumulated in deltaic and coastal plain settings, and contain both reservoir and source rocks. Widespread erosion and peneplanation in the Callovian coincided with continental breakup and the initiation of sea-floor spreading in the Argo Abyssal Plain.

From the Late Jurassic to the Cenozoic, accommodation space was controlled by the interplay of thermal subsidence, minor reactivation events and eustasy. Upper Jurassic interbedded sandstones and shales (Vulcan Formation) onlap and drape the pre-Middle Jurassic structures, providing a thin, regional seal, and potential source rocks across the basin (Figure 8, Figure 9 and Figure 10). An overall transgressive cycle began in the Early Cretaceous and peaked in the mid-Turonian, with open marine conditions established throughout the basin by the Aptian. Thick marine claystones deposited during this period (Echuca Shoals and Jamieson formations) provide a regional seal and contain potential source rocks, with particularly high total organic carbon (TOC) values recorded at the maximum flooding surfaces of several Early Cretaceous transgressive cycles (Blevin et al, 1998a).

The Turonian–Cenozoic succession represents a major progradational (regressive) cycle in which the shelf edge migrated northwestward to the outer limits of the Brecknock-Scott Reef Trend. The development of submarine canyons on the Yampi Shelf and deposition of turbidite mounds within the central Caswell Sub-basin occurred during the middle to late Campanian (Benson et al, 2004). Inversion commenced in the middle to late Miocene as a result of the convergence of the Australia-India and Eurasia plates (Shuster et al, 1998).

An outer sub-basin dry gas-prone system where the gas is reservoired within the Plover Formation and sourced from mixed terrestrial and marine organic matter deposited in a fluvio-deltaic environment. As documented by Le Poidevin et al. (2015), the driest accumulations are Torosa and Brecknock with condensate/gas ratios (CGR) of less than 20 bbl/MMscf (112 l/Mcm). The Brecknock South/Calliance accumulation has a slightly wetter CGR of 20–30 bbl/MMscf (112–168 l/Mcm). The Argus gas accumulation (CGR < 10 bbl/MMscf [56 l/Mcm]; Keall and Smith, 2004) probably represents a northern extension of this petroleum system, as do the adjacent discoveries at Kronos, Poseidon and Proteus. Similarly low CGR values are recorded in the Plover Formation of the Ichthys and Prelude accumulations (< 10 bbl/MMscf [56 l/Mcm]). Edwards et al (2004; 2014) proposed that the Lower–Middle Jurassic Plover Formation was the most likely source for these gases, whereas a Permo-Triassic source has been modelled by Belopolsky et al (2006).

A central sub-basin wet gas-prone system that is reservoired in the Brewster Member of the upper Vulcan Formation and includes the Ichthys and Prelude/Concerto accumulations (CGR = 60 bbl/MMscf [337 l/Mcm]). The interpreted condensate-rich gas at Echuca Shoals (Nexus Energy Ltd, 2014) and probably Burnside, represent an extension of this system. These accumulations are most likely sourced from within the marine Jurassic Vulcan Formation (Grosjean et al, 2015).

An inner sub-basin oil- and gas-prone petroleum system sourced from predominantly marine algal and bacterial organic matter within the Lower Cretaceous sediments of the Echuca Shoals Formation. The Cornea and Gwydion accumulations, the Caswell 2 oil accumulation and the Kalyptea 1 ST1 gas show belong to this petroleum system which Blevin et al (1998b) defined as the Westralian (W3) Petroleum System. The Cornea and Gwydion oils and gases vary in their degree of biodegradation.

Source rocks

A comprehensive assessment of the source rock potential of the Browse Basin was undertaken by Boreham et al (1997), and the results summarised by Blevin et al (1998a, 1998b). These studies recognised organic-rich rocks with fair to moderate oil potential at numerous stratigraphic levels within the Permian to Lower Cretaceous succession. Although many possible source units within this succession have liquid potential i.e. Hydrogen Index (HI) values of >200 mg hydrocarbons/gTOC, they contain less than 2% TOC. At these low-to-moderate TOC levels, any generated oil may not be expelled and could be subsequently cracked to gas at higher maturities.

Local, thin, high-quality coals and pro-delta shales with high source potential occur within the thick succession of Lower–Middle Jurassic Plover Formation sediments that extend throughout the Caswell Sub-basin and reach a maximum penetrated thickness within the Barcoo Sub-basin (920 m in Barcoo 1). This section is dominated by fluvio-deltaic sediments, including pro-delta shales and coastal plain shaly coals that have significant source potential (Blevin et al, 1998b). Hydrocarbons generated from this succession are likely to be dominated by gas rather than oil.

The Upper Jurassic Vulcan Formation is generally thin throughout the Browse Basin, with major sediment thickening occurring towards the Heywood Graben in the northeast, where restricted marine source facies are likely to be best developed. Localised thickening of Upper Jurassic sediments also occurs on the Leveque Shelf and Prudhoe Terrace, but here the section is dominated by deltaic facies with poorer quality terrigenous organic matter.

Thick marine claystones within the Lower Cretaceous Echuca Shoals and Jamieson formations occur within both the Caswell and Barcoo sub-basins and contain mixed marine and terrestrial organic matter with moderate to good source potential. However, available pyrolysis data suggest that these sediments have better liquid hydrocarbon potential in the Caswell Sub-basin (HI = 150–350 mg hydrocarbons/gTOC) than in the Barcoo Sub-basin (HI = 100–250 mg hydrocarbons/gTOC; Kennard et al, 2004).

Reservoirs and seals

Caswell Sub-basin

Exploration activity has focused on the Caswell Sub-basin, where the Upper Jurassic–Lower Cretaceous upper Vulcan and Lower Cretaceous Echuca Shoals and Jamieson formations form the regional seal. The thick (500–600 m) Callovian–Turonian claystone seal exceeds the throw of the faults within the underlying reservoirs, ensuring an adequate lateral seal across much of the basin. Sections within the lower Vulcan Formation also form adequate seals for Plover Formation reservoirs. Potential intraformational seals occur within the Plover Formation (Blevin et al, 1998b), while marls and mudstones provide potential seals for Campanian–Maastrichtian turbidites and unconfined fan sandstones in the Puffin Formation (Benson et al, 2004). The influence of basement controlled drainage patterns on the Kimberley Block has had a profound effect on the distribution of shelfal sedimentation of both reservoirs and seals (Tucker, 2009).

The Lower–Middle Jurassic Plover Formation and the Berriasian Brewster Member of the upper Vulcan Formation are the most laterally extensive reservoirs across the Caswell Sub-basin. Oil and gas shows also occur in locally developed sandstones of the Middle–Upper Jurassic Montara Formation, and in submarine fans and turbidites of Barremian, Campanian and Maastrichtian age (Le Poidevin et al, 2015).

The Ichthys, Prelude, Concerto and Mimia gas accumulations are collectively reservoired within the Plover, Montara and upper Vulcan (Brewster Member) formations. The Plover Formation at these locations comprises a fluvial-deltaic sandstone and mudstone succession with marine affinities towards the top. The main reservoir is characterised by a massive high-energy, cross-bedded channel sandstone. The top of the Plover Formation is associated with the regional Callovian unconformity; however, at Ichthys, the unconformity is poorly expressed with the estuarine to paralic sandstones of the Plover Formation passing into lagoonal and barrier island clastics of the ‘Ichthys Formation’, and then into the shoreface and shelfal sandstones of the basal Montara Formation and lower Vulcan Formation (Ban and Pitt, 2006; Le Poidevin et al, 2015). The Montara Formation is a thinner, secondary reservoir of localised extent, consisting of prograding fan-delta systems. The Brewster Member is a thick sequence of clean, relatively high net/gross sands that often exhibits poor to moderate reservoir properties. It was deposited by poorly confined, sand-rich, mid-slope grain flows or high-density turbidity currents on a deep-water ramp and hence also contains mudstones.

The Crux structure within the Heywood Graben in the northeastern Caswell Sub-basin hosts gas within the Upper Triassic Nome Formation and Lower–Middle Jurassic Plover Formation. The Nome Formation was deposited in a relatively high-energy fluvial environment and comprises quartz-rich, medium to very coarse grained sandstone with interbedded siltstone, mudstone and minor coal. Unlike those of the Nome Formation, the Plover Formation sandstones are fine to very fine grained and are interbedded with occasional medium to coarse grained sandstones, siltstones, claystones and minor coals.

Brecknock-Scott Reef Trend

At the Brecknock and Calliance accumulations, the upper Plover Formation is the main reservoir, while the lower Plover Formation acts as the main reservoir at Torosa. The older sandstones were deposited in a fluvial-dominated upper delta plain with the younger sandstones reflecting deposition in a more tidally influenced lower delta plain environment. Tuffaceous volcanics are also present at some locations within the lower Plover Formation (Tovaglieri et al, 2013).

Yampi Shelf

The Cornea oil and gas accumulation on the Yampi Shelf is reservoired within the Cretaceous Heywood Formation, in which fine to very fine grained sandstones of Albian age were deposited in a lower to upper shoreface environment. The primary seal is a marine Albian claystone; however, it shows elevated gas readings at some locations and only provides a partial seal.

Modelling suggests significant quantities of gas were expelled from the Plover Formation throughout the Browse Basin, including the southern, outer and northeastern Caswell Sub-basin, Seringapatam Sub-basin and parts of the Barcoo Sub-basin. This gas expulsion occurred during the late Early–Late Cretaceous and Neogene. Further to the northeast, oil was expelled from Jurassic sediments in the Plover Formation in the Heywood Graben during the Paleogene and Neogene and the lower Vulcan Formation during the Neogene (Kennard et al, 2004). Fluid inclusion analysis indicates that these Jurassic sediments are the likely source of the thick palaeo-oil columns interpreted in Heywood 1 and Crux 1 (Eadington and Middleton, 2000; Brincat et al, 2004). Lesser quantities of oil are modelled to have been expelled from the Vulcan Formation in the central and southern Caswell Sub-basin. An investigation of the fluid inclusions in the gas reservoirs of the Browse Basin has shown that the hydrocarbon charge consisted of an early oil charge, filling only the crests of the structures before being either displaced or absorbed by gas (Brincat and Kennard, 2004). Only relatively minor gas expulsion, but no oil, is predicted to have occurred in the Barcoo Sub-basin where source rocks are generally leaner (Kennard et al, 2004). If the Jurassic units of the Seringapatam Sub-basin contain good quality source rocks, then significant quantities of oil and gas could have been expelled during the Paleogene (Kennard et al, 2004).

Hydrocarbon generation and expulsion studies of Lower Cretaceous (Echuca Shoals and Jamieson formations) source rocks using Small Angle Neutron Scattering (SANS), confirms the existence of potential source rocks that are sufficiently thermally mature to generate both oil and gas, but which show either little or no evidence of expulsion or effective regional charge (Radlinski et al, 2004). Similarly, fluid inclusion analysis provides no evidence of an effective regional oil charge of Cretaceous reservoirs in the Caswell Sub-basin (Brincat and Kennard, 2004; Brincat et al, 2004). However, because the organic-rich sediments within this succession occur as thin transgressive sheets deposited on a gently inclined ramp margin in response to fluctuating sea level, detailed understanding of the local expulsion-migration history requires higher resolution (systems tract level) sequence stratigraphic models. Effective oil charge from parts of the Echuca Shoals Formation is confirmed by geochemical analysis of the Cornea, Gwydion and Caswell accumulations (Boreham et al, 1997; Edwards & Zumberge, 2005), and is postulated as the probable source of the inferred gas accumulation at Marabou 1 ST1 (Benson et al, 2004) and Adele 1 (Grosjean et al, 2015).

Exploration history

The Browse Basin is one of the richest hydrocarbon-bearing basins in Australia. The Caswell Sub-basin and the Leveque and Yampi shelves lie in shallow to intermediate water depths and are mature in exploration terms, hosting significant gas accumulations and discoveries of gas and, to a lesser extent, oil. This contrasts with the Barcoo Sub-basin and deep water Scott Plateau and Seringapatam Sub-basin, which are underexplored. Over 140 wells have been drilled in the basin and over 180 000 km of 2D and 46 000 km2 of 3D seismic data has been acquired, some of which is now open file (Department of Mines and Petroleum, 2014).

Exploration commenced in the Browse Basin in 1967, when the North West Shelf Joint Venture acquired 1 600 km of regional seismic data (Department of Mines and Petroleum, 2014). The first well drilled in the Browse Basin was Leveque 1 (1970), a stratigraphic test of the sedimentary succession on the Leveque Shelf. This was followed by the discovery of gas at Scott Reef 1 in 1971. This well intersected a thick sequence of gas-bearing reservoirs within Lower–Middle Jurassic Plover Formation sandstones and Upper Triassic sandy dolostones of the Nome Formation on the southern culmination of a faulted anticline (Willis, 1988). Two appraisal wells, Scott Reef 2A in 1977 and North Scott Reef 1 in 1982, were drilled to further delineate the extent of the accumulation (Bint, 1988). No net hydrocarbon pay was assigned to the Scott Reef 2A well, but North Scott Reef 1 encountered a Jurassic hydrocarbon reservoir with an inferred net thickness of 122.9–134.2 m. In 1979, Brecknock 1 tested a broad anticlinal feature 40 km southwest of Scott Reef. The well penetrated 68.3 m of net gas sandstone in Lower to Middle Jurassic sediments (Bint, 1988).

Evidence of the oil potential of the basin was demonstrated by the Gwydion 1 (1995) and Cornea 1 (1997) oil and gas discoveries, both located on the Yampi Shelf. These discoveries challenged the previous perception that the basin was gas-prone (Stein et al, 1998). Gwydion 1 intersected three gas-bearing zones and one oil and gas-bearing zone in Lower Cretaceous (Barremian to Albian) shallow marine sandstones draped over a prominent basement high (Spry and Ward, 1997). The Cornea 1, 1B and 2 wells encountered a 25 m gas column overlying an 18 m oil column in a lower Cretaceous (Albian) reservoir (Ingram et al, 2000), and were followed by nearby oil occurrences at Cornea South 1 and 2 ST1, Focus 1 and Sparkle 1, and gas at Stirrup 1 and Macula 1 throughout 1998. In the same year, Psepotus 1 and Caspar 1A discovered small gas accumulations within Lower Cretaceous sandstones on the Leveque Shelf and Yampi Shelf, respectively. In the Caswell Sub-basin, Adele 1 (1998) discovered gas in the lower Jamieson and Echuca Shoals formation sandstones and Columba 1A ST1 (1999) discovered gas in the upper Vulcan Formation.

Drilling in 2000 resulted in the discovery of several major gas accumulations, as well as the extension of previously recognised gas provinces. These included Brecknock South 1, located on the Brecknock-Scott Reef Trend 19 km south of Brecknock 1, and Argus 1 to the north of this trend. Further drilling on the Brewster structure, including the Titanichthys 1, Gorgonichthys 1 and Dinichthys 1 wells, better defined the extent of gas within the Plover (Ichthys) and Montara formations, as well as the Brewster Member of the Vulcan Formation (Ban & Pitt, 2006). Crux 1, drilled in the Heywood Graben in the northeastern Caswell Sub-basin, encountered a 280 m gross gas column in the Upper Triassic Nome Formation, with secondary reservoirs being found within the Plover and Montara formations (Kaoru et al, 2004).

Evaluation of the gas accumulations along the Brecknock-Scott Reef Trend also continued in 2005-09 with Woodside drilling the extension/appraisal wells Torosa 1, 2, 3, 4, 5 and 6, Brecknock 2, 3 and 4, and Calliance 1, 2 and 3, as well as the Snarf 1 exploration well on the edge of the Caswell Sub-basin close to the Seringapatam Sub-basin.

In 2012, ConocoPhillips (Browse Basin) Pty Ltd and Karoon Gas Australia Ltd commenced phase 2 of their joint venture in the Browse Basin, embarking on an exploration program to evaluate the gas resources of the Greater Poseidon Trend (Karoon Gas Australia Ltd, 2012). The first well, Boreas 1, flowed gas from the primary Plover Formation reservoir (Karoon Gas Australia Ltd, 2012). Their second well, Zephyros 1, was completed in March 2013. In this well, 108 m of core was cut through gas-bearing sandstones that were interpreted to have high mobility values (Karoon Gas Australia Ltd, 2013a). This discovery was followed by the drilling of Proteus 1 ST1 in the same permit. Wireline logging indicated multiple gas-charged reservoirs within the Jurassic, with an 87 m gross reservoir with high net pay (Karoon Gas Australia Ltd, 2013b). The sidetrack measured flow rates of up to 7.3 MMscf/d (0.207 MMcm/d) through a 16/64” choke at 4 457 psi (30 730 kPa) and condensate gas ratios of 19–22 bbls/MMscf. Production wells are predicted to flow at commercial rates in excess of 100 MMscf/d (2.831 MMcm/d; Karoon Gas Australia Ltd, 2013c). The fourth well in the phase 2 drilling campaign, Grace 1, was plugged and abandoned in January 2014, with no significant hydrocarbons encountered (Karoon Gas Australia Ltd, 2014a). The campaign continued in March 2014 with the drilling of Poseidon North 1, which encountered Jurassic gas-bearing sands across a 20 m gross, 12 m net reservoir interval, though pressure data was inconclusive. (Karoon Gas Australia Ltd, 2014b). The sixth and final well in the phase 2 drilling campaign was Pharos 1, spudded in May 2014. The discovery of movable hydrocarbons in the gas-charged Montara Formation across a 53 m gross interval with 34 m interpreted net pay was announced in July 2014 (Karoon Gas Australia Ltd, 2014c).

In 2012, Santos Ltd spudded Crown 1 in the Caswell Sub-basin (Santos Ltd, 2012a), and announced the discovery of a 61 m net gas pay in the Jurassic Montara and Plover reservoirs in November 2012 (Santos Ltd, 2012b). Bassett West 1, operated by joint venture partner Total E & P, was spudded on 17 December 2012, followed by the announcement in June 2013 of a 7.5 m gas pay in Jurassic sandstones (Santos Ltd, 2013a). Dufresne 1 was spudded in June 2013, to target Jurassic gas (Santos Ltd, 2013b, 2013c) but the well was subsequently plugged and abandoned (Santos Ltd, 2013d).

In August 2014, Santos made a discovery in the Lasseter 1 well, with 78 m confirmed net gas/condensate pay in the Jurassic lower Vulcan and Plover formations (Santos Ltd, 2014). Two other exploration wells were drilled in 2014; Hunt Oil spudded Schooner 1, its first Australian operated well, in March 2014, with no published results at this time. Pryderi 1 was spudded by CalEnergy on the Yampi Shelf in October 2014 and was dry with some residual oils shows (Energy-pedia, 2014).

A recent hydrocarbon discovery was made in 2015 by Shell drilling West 1 (Auriga) in the northern Browse Basin (OE Digital, 2015).

Three seismic surveys were conducted in the Browse Basin during 2014. CGG proposed Phase II of the Schild MC3D survey consisting of ~16 650 km2 to run throughout 2014 (CGG 2013), following on from the 1 441 km2 from Phase I, conducted by Fugro in 2013 (Fugro, 2013). Woodside’s Lord 3D marine seismic survey, conducted during April-June 2014, covered 3 352 km2 of Petroleum Exploration Permit WA-495-P on the Scott Plateau and western Barcoo Sub-basin, as well a small portion of the northern Roebuck Basin (Woodside, 2014b). Petroleum Geo-services (PGS) acquired the ~15 000 km2 Caswell MC3D marine seismic survey in the Caswell Sub-basin in June 2013 to 2014 (Petroleum GeoServices, 2013). IPB is planning the acquisition of 600 km2 3D seismic survey in WA-471-P for completion by the 3 May 2016 (Offshore Energy Today, 2015) and a 900 km2 3D seismic survey in WA-485-P by the 13 May 2016 (IPB, 2015).

During October and November of 2014, Geoscience Australia conducted a marine survey in the Caswell Sub-basin, including outer portions of the Yampi and Leveque shelves, to collect data to support a CO2 storage assessment as part of the National CO2 Infrastructure Plan (NCIP). In addition to acquiring geological, water column and seabed habitat data the survey aimed to identify and sample features indicative of active or extinct natural fluid seepage (Geoscience Australia, 2014). The post-survey report will shortly be published by Howard et al. (2016) and the petroleum potential and carbon capture and storage report is in preparation (Rollet et al., In prep).

Development status

In the Browse Basin there are a number of projects in various stages of planning and development: the Ichthys LNG Project and the Prelude FLNG project are under construction while the Browse FLNG proposal is in planning.

Ichthys LNG Project — This Project (Production Licences WA-50-L and WA-51-L) is operated by INPEX (62.3%) with joint venture partners Total E & P Australia (30%), Tokyo Gas (1.6%), Osaka Gas (1.2%), Chubu Electric (0.7%), Toho Gas (0.4%), CPC Corporation (2.6%) and Kansai Electric Power Australia (1.2%). INPEX Browse Ltd (INPEX) announced the final investment decision (FID) in January 2012 for an offshore central processing facility (CFP) and a floating production, storage and offloading (FPSO) facility, connected by an 889 km gas pipeline to an 8.9 million tonnes per annum (mtpa) liquefaction plant and export terminal in Darwin. The project is expected to produce first gas by 2017, with a life of 40 years (INPEX, 2014), and will also produce condensate and LPG. Resource estimates indicate 12.8 Tcf (362.5 Bcm) of gas and 527 MMbbl (83.8 Gl) of condensate over 40 years (INPEX 2014). The pipeline was recently completed (ABC, 2015).

Prelude FLNG — Shell (67.5%) is the operator of Production Licence WA-371-P with INPEX (17.5%), KOGAS Prelude Pty Ltd (10%) and OPIC Australia Pty Ltd (5%) in which the Prelude and Concerto gas accumulations occur. Shell Development Australia Pty Ltd (Shell) announced the FID for a 3.6 mtpa Floating LNG (FLNG) project in May 2011 for production from the Prelude and Concerto gas accumulations. The floating facility will be 488 m long and 74 m wide, and when fully loaded will displace about 600 000 tonnes. The FLNG vessel is currently in the construction phase with first gas due in 2017. The Prelude accumulation is situated in the northeastern area of the Ichthys accumulation (Figure 2) and will also produce condensate (Shell Australia Ltd, 2014). The Prelude FLNG facility is anticipated to operate for 25 years, producing 3.6 mtpa of LNG, 1.3 mtpa of condensate and 0.4 mtpa of LPG (Shell Australia Ltd, 2015a). The Crux field is also likely to become part of this development (Offshore Energy Today, 2012). The Retention Lease AC/RL9, containing the Crux accumulation, was granted in February 2013. Titleholders include the operator Shell (82%) and its JV partners SGH Energy (15% - who acquired Nexus in 2014/15) and Osaka Gas (3%). The Crux accumulation contains resources of 2.2 Tcf (62 Bcm) gas and 74 MMbbl (11.8 Gl) liquids and a five year work program has been defined. A report outlining the final development concepts is due in year 5 (Nexus Energy Ltd, 2013). Shell has recently drilled Auriga West 1 (2015) immediately west of this accumulation.

Browse FLNG— Operated by Woodside with joint venture partners Shell, BP Developments Australia Pty Ltd, Japan Australia LNG (MIMI Browse) Pty Ltd and PetroChina International Investment (Australia) Pty Ltd (joint venture percentage holdings vary due to multi-permits). Woodside Petroleum Limited (Woodside) announced the Browse Joint Venture’s intention to commercialise the Torosa, Brecknock and Calliance gas accumulations via a FLNG project in September 2013 in retention leases WA-28-R, WA-29-R, WA-30-R, WA-31-R and WA-32-R. As of June 2015, the Browse Joint Venture agreed to enter the front-end engineering and design (FEED) phase for the development. A final investment decision was made in early 2016 and the project has been shelved. The estimated resources are 15.4 Tcf of dry gas and 453 MMbbl of condensate (Woodside, 2015).

Other gas fields include Argus, Burnside, Crown/Proteus and Lasseter in the Caswell Sub-basin. Significant gas discoveries include: Abalone 1 ST1, Adele 1, Bassett West 1, Boreas 1, Burnside 1 ST1, Columba 1A/ST1, Echuca Shoals 1, Kronos 1, Marabou 1 ST1, Mimia 1, Octans 1 and Poseidon 1 and 2. Libra 1 and Octans 1 are satellite fields to the Crux accumulation in the Heywood Graben (La Poidevin et al, 2015). The Hippolyte 1 gas discovery is on trend with the Crux structure (Le Poidevin et al, 2015). The gas discovery at Psepotus 1 and gas shows at Leveque 1 are located on the Leveque Shelf. Oil discoveries are focused on the Yampi Shelf, although an oil and gas field has been discovered at Caswell within the Caswell Sub-basin. An oil and gas field has also been identified at Cornea on the Yampi Shelf. Other discoveries include Caspar 1A (gas), Focus 1 (oil and gas) and Gwydion 1 (oil and gas).

Marine and environmental information

Large areas of the Browse Basin are unexplored, and seabed information is not readily available. However, information available indicates that he marine environment overlying the Browse Basin is primarily continental shelf and slope with isolated reefs located offshore from the Kimberley region. This tropical monsoonal-influenced area is characterised by seasonally modified warm coastal and offshore currents, macro-tides, and tropical cyclones.

Climate of the region

The northwest shelf region experiences a dry (arid tropical) climate with two distinct seasons: the northwest (summer) monsoon (October-March) and, the northeast to southeast (winter) monsoon (April-September), with a rapid transitional period between each season. On average, approximately 90% of the annual mean rainfall (779.2 mm) occurs in the period between December and March and is associated with storm activity. Winds during the summer monsoon are typically westerly/northwesterly and humid, while during the winter monsoon winds are typically drier southeasterlies which originate over the Australian mainland. The monthly maximum and minimum temperatures recorded at Browse Island in 2014 were 34.1°C and 21.6°C. Offshore, water temperatures in reef lagoons are commonly 28-30°C, while during strong La Nina events, sea surface temperatures may reach 34°C. The region is subject to cyclone activity, and the cyclone season officially runs from November to May, with approximately five tropical cyclones occurring per year on average. Highest cyclone activity in the region typically occurs during March and April. Between 1996/97 and 2006/07, 17 tropical cyclones passed within 200 km of Browse Island.

Oceanic Regime

Oceanic circulation in the Browse Basin is strongly influenced by the southward flowing, offshore, Indonesian Throughflow, and by the Leeuwin Current along the northwest shelf edge (Condie and Andrewartha, 2008). The surface water above the Caswell and Barcoo Sub-basins, together with Scott Plateau and the Seringapatam Sub-basin form part of the zone over which the Indonesian Throughflow passes. The warm, low salinity, southwesterly directed currents of the Indonesian Flowthrough joins the westward directed South Equatorial Current (Adamo et al, 2009; Kuhnt et al, 2004). This surface current transports warm equatorial water south-eastwards towards the Wharton Basin. At shelf break on the Yampi and Leveque shelves, the seasonal (Autumn) Holloway Current transports warm water, heated on the northwest shelf, southwards (Adamo et al, 2009). The Holloway Current begins to flow at the termination of the northwest monsoon season. Between the Indonesian Throughflow and the Holloway Current, an anticlockwise surface gyre exists.

The northwest shelf and the Browse Basin region is macrotidal, and inshore is ebb-tide dominated. The Kimberley Coast experiences some of the largest tides in the Southern Hemisphere. Tidal ranges generally exceed 4 m on the Leveque Shelf; tidal ranges of up to 10 m occur at Broome, and up to 12.5 m in King Sound. Tides at Broome are semi-diurnal (two high tides, and two low tides per day). On the outer shelf, the ebb and flood tidal currents are orthogonal to the coast (James et al, 2004b) whereas on the inner shelf, flood tidal currents run parallel to the coast. Tidal currents are strongest around the shelf break (James et al, 2004b). However, tidal currents play a significant role in nearshore and coastal sediment transport; e.g. in King Sound, tides mobilise significant areas of medium-grained sand (250–500 µm diameter) 100% of the time (Porter-Smith et al, 2004).

The Browse Basin and Kimberley coastal region are characterised by large internal waves, internal tides and strong seasonality in the near-surface ocean circulation (McKinnon et al, 2015). Internal waves occur around shelf break (Holloway, 1987), with resulting internal tides present across the continental slope from approximately 100 m to 1 000 m depths (Holloway, 2001). Internal waves with 60 m amplitudes have been observed to form around Scott Reef, located in water depths of ~ 500 m on the continental slope of the northwest shelf (Wolanski and Deleersnijder, 1998).

Seabed environments: regional overview

The Browse Basin is located adjacent to the Australian coast within the northwest marine region. It extends from shallow shelf waters adjacent to the Kimberley coast of Australia, to near-abyssal depths on the western margin of Scott Plateau. At the basin scale, the Yampi and Leveque Shelves form terraced and plateaued seabed on the eastern margin of the main Browse Basin depocentres of the Caswell and Barcoo sub-basins (Harris et al, 2003). The width of the shelf is greatest (~ 250 km) across the Yampi Shelf-Prudhoe Terrace to Scott Reef transect.

The edge of the Yampi Shelf and the transition to the Caswell Sub-basin occurs in water depths of 100-130 m on the Prudhoe Terrace, with most of the Caswell Sub-basin lying in water depths of 200-500 m. The western margin of the Caswell Sub-basin marks a transition from shelf to slope settings. Where there are shelf-edge reefs, the transition to deeper water is sharp. However, the transition to deep water across inter-reefal areas from shelf to slope, is significantly more gradual. The majority of the Scott Plateau is composed of terrace and plateau, in water depths of 2 000-3 000 m, while its western slope occurs between 3 000 m and 5 000 m depth. The plateaued Seringapatam Sub-basin mostly lies in water depths of 1 700-2 300 m. Several submarine canyons are present along the Caswell to Seringapatam Sub-basin transition, and on the western slope of Scott Plateau where it abuts the Argo Abyssal Plain.

Islands and Reefs

The Kimberley-Browse coastal and offshore region is a poorly studied area (McKinnon et al, 2015) of high conservation value, consisting of emergent oceanic reefs, submerged oceanic shoals, fringing coastal coral reefs, mangroves, tidal mudflats and a turbulent water column. The principal islands and reefs in the Browse region include Browse Island, Echucha Shoal, Heywood Shoal, Scott Reef, Adele Island, Seringapatam Reef and Lynher Reef. Additional shoals include, Vulcan Shoal, Goeree Shoal, Eugene McDermott Shoal. Compared with the generally high tidal energy environment of the northwest shelf, reefs provide shallow water environment with comparatively low energy regimes. At North Scott Reef and elsewhere, coral reef has developed since the early Holocene over previously exposed reefal lithologies from the last interglacial, following the post-glacial rise in sea-level (Collins, 2011, 2014). Shallow-water reefs such as Scott Reef are prone to damage during tropical cyclones (e.g. Cyclone Fay of 2004). Surficial sediment on the northwest shelf is predominantly carbonate. The northwest shelf (< 300 m depth) is an oceanic carbonate ramp, similar in scale to the Great Bahama Bank (James et al, 2004, Belde et al, 2014), and hosts a large number of coral reefs. These include isolated oceanic reefs (Ashmore Reef, Seringapatam and Scott Reefs, Rowley Shoals), and the island-associated shelf reefs of the Kimberley coast and Dampier archipelago; Pilbara reefs (Barrow and Montebello Islands); and Ningaloo Reef (Collins, 2011, 2002; Collins and Testa, 2010; Wilson, 2013). The seabed is predominantly composed of unconsolidated sediment on the continental shelf. This consists of medium- and coarse-grained CaCO3-rich (~ 90%) sands and gravels derived from relict fossiliferous carbonate material, with minor authigenic phosphate and glauconite components (Collins, 2011). Between 200 m and 500 m water depths, hard seabed and sandy carbonate sediments are dominant. In deeper water, particularly in areas distal to reefs, the seabed is dominated by fine and fine-medium grained unconsolidated sediments (Kuhnt et al, 2006). There are however, also areas of seabed in these deeper settings where hard rock is present. In the western Caswell Sub-basin large-scale sediment bedforms indicate the presence of strong southeast – northwest oriented bottom-flow currents. These are potentially related to internal tides (Belde et al, 2014).

Along the Yampi and Leveque shelves, sediments are palimpsest, and of mixed age, with little modern material being deposited. Sediment here is either sourced from the post-mortem deposition of skeletal carbonate or in places sourced from the coast and transported across the shelf. However, at the coast, the transport of modern terrigenous sediment, where it exists, is generally in a shore-parallel direction.

The continental shelves of the Yampi Shelf, Prudhoe Terrace and Leveque Shelf, located in a high-energy, shallow water, tropical setting, have a much thinner sediment cover than the centrally located Caswell Sub-basin (Stephenson and Cadman, 1994). Hardgrounds are present at several points along this sediment-starved margin (Rollet et al, 2006). Vibrocores recovered on the Leveque Shelf up to 2.5 m long, bottom out on hard substrates (Picard et al, 2014). These cores were recovered from an area with elongate, shore-normal banks and ridges, carbonate terraces, channels and large bedforms up to 8 m high dominated by coarse sediment.

Recent geological history

The modern geomorphic characteristics of the Browse Basin are inherited from pre-existing structures. During the Quaternary, these were further developed particularly through reef growth and differential sedimentation during sea-level oscillations of up to 130 m. Modern reefs are generally present on pre-existing palaeo-reefal structures. Since the onset of the middle Pleistocene (Ionian Age) (Lisiecki and Raymo, 2005; Gibbard et al, 2009; Head et al, 2008), glacially low sea levels would have exposed the Yampi and Leveque Shelves, and Prudhoe Terrace to approximately 130 m below present level, with remnant glacial coastlines existing in the vicinity of Browse Island, in the Caswell Sub-basin. Areas including the Cornea site would have been sub-aerially exposed for moderate amounts of time since the middle Pleistocene. A prominent terrace-edge scarp at -125 m on the Leveque shelf has been attributed to formation during glacial low sea levels, the most recent being 23-19 ka BP (Collins, 2011; James et al, 2004). It has been suggested that subsidence has been occurring on the northwest shelf since at least the last interglacial, ~125 ka BP (Collins and Testa, 2010).

Seabed seepage

In the Browse Basin, hydrocarbon seeps have been observed at Cornea and Heywood Shoals on the northern Yampi Shelf (Logan, 2010; Jones et al, 2005; Rollet et al, 2006). Pockmarks have also been noted in the Oobagooma Sub-basin and Broome Platform of the offshore Canning Basin, southwest of the Leveque Shelf (Jones et al, 2007, 2009). Pockmark formation at these latter locations has been attributed to the expulsion of seawater at the seabed, driven by tidal pumping through the shallow sub-surface. Recently, small pockmarks associated with the breakdown of organic matter in the near-surface have been observed at the seabed in the northeast of the Caswell Sub-basin (Howard et al, in press). Pockmarks similar to those observed by Jones et al (2007) are present on the Leveque Shelf (Picard et al, 2014).

Fauna and Flora

Browse Island is an internationally important reserve for breeding seabirds and migratory shorebirds, with all species variously listed under the EPBC Act 1999 (Clarke, 2010). The North West Slope Trawl, Western Tuna and Billfish Commonwealth Fisheries, and the WA Pearl Oyster Fishery are also in the area.

Commonwealth Marine Reserves

Kimberley Commonwealth Marine Reserve

The Kimberley Commonwealth Marine Reserve provides protection for the communities and habitats of waters offshore of the Kimberley coastline, ranging in depth from < 15 m to 800 m. Continental shelf, slope, plateau, pinnacle, terrace, banks and shoals and deep depressions/valley seafloor features are represented in this reserve. The reserve includes examples of the communities and seafloor habitats of the Northwest Shelf Transition, Northwest Shelf Province and Timor Province provincial bioregions, along with the Kimberley, Canning, Northwest Shelf and Oceanic Shoals meso-scale bioregions.

Data Gaps

Most of the Caswell and Barcoo Sub-basins, apart from sparse data from marine surveys, remain relatively unexplored. Sub-surface data from the major reefs are generally only available from regional seismic studies. Detailed seabed characteristics are only available for water depths < 300 m (Collins, 2011).

Seismic line BBHR07 across the central Browse Basin including parts of the the Caswell Sub-basin and Yampi Shelf. Interpretation by Geoscience Australia. Location of the seismic section is shown in Figure 3

Seismic line nsw07ph2/32 across the northern Browse Basin including parts of the Seringapatam and Caswell sub-basins. Seismic line courtesy of PGS. Interpretation by Geoscience Australia. Location of the seismic section is shown in Figure 3

The department acknowledges the traditional owners of the country throughout Australia and their continuing connection to land, sea and community. We pay our respect to them and their cultures and to the elders past and present.